## Why
Concurrent Linux test spawns can inherit writable descriptors for executable fixtures, preventing those fixtures from being executed even after the writing test closes its descriptor.
## What changed
Replace direct executable fixture writes and copies across the Rust tests with `codex_utils_cargo_bin::write_executable` and `copy_executable`. These existing helpers complete writes and copies in separate processes on Linux.
Remove the empty temporary inode before creating fake bubblewrap executables, and stage tampered bubblewrap bytes in a separate data file before copying them into the executable fixture. Add the required test dependencies.
GitOrigin-RevId: c875c83088731a4933ef1574e8d79d8cecc12f54
## Why
Unix command hooks use a `pre_exec` callback to detach from the controlling terminal, forcing the launcher to fork. Native spawn attributes can create the detached session without that callback.
## What changed
- Route Unix hooks through `codex_utils_pty::Command` with `ProcessMode::NewSession`.
- Extend the native `posix_spawn` launcher to detached Linux sessions, retaining compatibility fallbacks for unsupported libc features and executable handling.
- Preserve the session environment snapshot, hook overrides, and restricted-variable filtering.
## Testing
Add regression coverage for launches without parent forks, terminal detachment, environment and argument handling, Linux `PATH` lookup, and executable compatibility. Cover process-group cleanup on cancellation and timeout, including descendants holding output pipes open, and preservation of background descendants after completed hooks.
GitOrigin-RevId: dbe2cf12d84aa79868cfb55ace40c7d5b5282703
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